Inverters that operate from a dc source and have a plurality of semiconductors that are periodically rendered conductive and non-conductive to supply ac current to a load, are susceptible to a type of fault referred to herein as "shoot-through." A shoot-through fault is one in which an abnormal combination of the inverter's semiconductors are simultaneously conducting so that they almost completely short circuit the dc input of the inverter. In many systems the inverter semiconductors include both main semiconductors for carrying load current and commutation semiconductors. Various circuit techniques have been employed in the past to prevent the shooting-through inverter semiconductors from being destroyed by heating due to the excessive current that they conduct during a shoot-through fault.
One such protection technique is to provide switchable shunt paths for diverting at least some of the current from the shooting-through semiconductors, the shunt paths being switched into conduction when a fault sensor detects a fault and produces a fault indication signal. Some of the shunt current paths that have been employed in the prior art include a separate "crowbar" semiconductor connected across the dc input terminals of the inverter, commutation semiconductors which in the absence of a fault are employed for commutation of the main semiconductors and which are all triggered into conduction upon occurrence of the fault indication signal, and other ones of the main semiconductors that are switched into conduction en masse upon occurrence of the fault indication signal to help share the fault current. Often, an impedance is connected in series with one of the dc supply lines to limit the total fault current.
The prior art also includes fault interruption circuitry by which an attempt is automatically made to stop the fault by momentarily applying a reverse voltage to the dc input terminals of the inverter to back-bias the inverter semiconductors, whereby their conduction is stopped. One of the fault interrupting circuits that has been used for applying a reverse voltage to the dc input terminals includes a capacitor that is charged in advance of the fault to a "forward" polarity by current flowing into the capacitor while the dc input terminals have normal dc voltage. Upon a shoot-through fault, the voltage of the dc input terminals becomes very low and the capacitance discharges its current through an inductor connected in series with the capacitor, and through whatever current paths may be available, including the shooting-through inverter semiconductors and any shunt current-diverting paths that may have been additionally provided to protect the shooting-through inverter semiconductors.
When the charge on the capacitor has diminished to zero as a result of its discharge current, the capacitor current does not cease because the inductor then produces a voltage of such polarity as to cause the current to continue to flow and to charge the capacitor with reverse polarity. The reverse polarity of voltage on the capacitor is applied to the dc input terminals of the inverter so as to back-bias the inverter semiconductors, and incidentally sometime to back-bias also the additional current-diverting paths, in an attempt to clear the shoot-through fault. Reference is made for purposes of background to patent application Ser. No. 306,521 entitled, "Fuseless Inverter" and filed Nov. 15, 1972, by Thomas J. Bernhardt and Frank N. Klein, and to U.S. Pat. No. 3,321,697 to Etter, issued May 23, 1967.
In apparatus of the prior art, some of the energy initially stored in the fault clearing capacitor has been dissipated unproductively and even counter-productively because some charge from the capacitor flows through the shooting-through inverter semiconductors before the fault sensor has had sufficient time to recognize the fault and to trigger the shunt current diverting paths into conduction. This early discharge from the capacitor not only diminishes the amount of energy available for subsequently reverse-biasing the inverter semiconductors, but also causes additional heating of the already overloaded inverter semiconductors that are shooting through.